Methods for warning motor vehicle operators at highway-rail grade rail crossings are either passive or active. Passive warning methods at public crossings are often required by law to include the statutory crossbuck sign posted for each direction of traffic traversing the tracks. Alternative signs may be posted in addition to the crossbuck sign, such as number of tracks signs, “Do Not Stop on Tracks” signs, “Look for Trains” signs, statutory yield signs, statutory stop signs, and railroad crossing advance warning signs. The roadway surface can be painted with stop bars and railroad crossing symbols. Warning devices at private roadway crossings of railroad tracks can be provided by the roadway owner or the railroad and may be absent altogether or can be any combination of passive or active devices identical to those used at public crossings or of unique design. Active warning devices, by example, can be a warning bell, flashing red lights, swinging red lights, gate arms that obstruct roadway vehicle lanes, solid or flashing yellow advance warning lights in combination with statutory crossbuck signs, number of tracks signs, railroad advance warning signs, various informational signs, and pavement markings. Historically it has been cost prohibitive to include active warning systems at every grade crossing, thereby limiting many grade crossings to merely passive warning systems.
Conventional railway systems often employ a method that uses track rails as part of a signal transmission path to detect the existence of a train within a defined length or configuration of track, commonly referred to as track circuits. The track rails within the track circuit are often an inherent element of the design of the circuit because they provide the current path necessary to discriminate the condition of the track circuit which is the basis of train detection.
A conventional track circuit is often based upon a series battery circuit. A battery, commonly referred to as a track battery, is often connected to one end of the track circuit and a relay, commonly referred to as a track relay, is connected to the other end of the track circuit. Current from the track battery flows through one rail of the track circuit, through the coil of the track relay and back to the track battery through the other rail of the track circuit. As long as all elements of this system are connected, the track relay will be energized. Typically, an energized track relay corresponds to the unoccupied state of the system in which a train is not present within the track circuit. In the event that a train does occupy the track circuit, the series track battery-track rails-track relay circuit becomes a parallel circuit in which the wheels and axles of the train provide a parallel path for current flow between the two track rails of the circuit. Most current flows in this new circuit path because its resistance is very low compared to the track relay resistance. As a result, the track relay cannot be energized if a train occupies the rails between the track battery and the track relay. A significant advantage of this system is that if the current path between the track battery and the track relay is opened, the track relay will not be energized. Common causes of track circuit failure with typical railroad fail-safe circuits that may interrupt the current path include a broken rail, broken wire connections between the battery or relay and the rail, broken rail joint electrical bonds, and failed battery power. Should any element of the circuit fail, the signal control element, typically the track relay, will revert to the safest condition, which is de-energized. The typical track circuit is also an example of railroad signal closed circuit design. All elements of the circuit are necessary and only one current path is available to energize the track relay.
The track battery/relay circuit is often the basic functional unit for railroad signal system design. The energy state of track relays provides the fundamental input to the logical devices that control automatic signal systems, including wayside train signal, crossing signal, and interlocking operation.
Previously known methods for detecting trains that approach highway-rail grade crossings monitor and compare track circuit impedance to a known audio frequency signal. The signal is continuously monitored by the train detection unit which is tuned to an unoccupied track (normal state) during installation. Signal strength and phase within certain limits produce an energized output that corresponds to an unoccupied track circuit. When signal strength and/or phase are not within the normal state limits the train detection unit output corresponds to an occupied track circuit. A train occupying the track circuit changes the impedance of the circuit. The change vector for a moving train correlates to position of the leading or trailing wheels and axle of the train in the track circuit, train direction and speed.
The most advanced of such devices are capable of providing a “constant warning time” control for highway grade crossing signal operation. One of the advantages of this method at its most advanced application is the ability to cause crossing signals to operate for a predetermined time prior to the arrival of a train at a crossing roadway regardless of train speed. This device may provide multiple, independently programmable outputs which may be used control separate and independent systems. One output can be programmed to control the actual operation of the railroad crossing signal and the second output can be programmed to provide the appropriate input to a separate traffic light system that governs motor vehicle movement at an intersection near the railroad crossing.
In one aspect, a vehicle detection system detects roadway vehicles and an action is taken. Often the action taken is to adjust the frequency of intersection light operation in response to changing traffic patterns. It is common that roadway conditions can change dramatically as a result of a traffic accident, draw-bridge operation, or a train passing. As a result the rate of speed for the roadway vehicles is dramatically reduced, and often stopped. The slow rate of speed and common stoppage of traffic commonly is not accurately detected by certain magnetic field detectors.
In another aspect of vehicle detection systems, trains are detected and active railroad signal crossing warning devices are activated to warn traffic at highway-rail grade crossings, and therefore advanced preemption of the warning devices is necessary. However, a major disadvantage to the use of known loop detectors is that they do not reliably detect slow-moving objects passing through the magnetic field. It is often the case that railroads require trains to stop for periods of time. Due to the size and mass of trains they do not have the ability to accelerate quickly from a stopped position. Therefore it is often the case that trains move at a slow rate of speed. One of the inherent problems associated with certain magnetic field detectors is that a requisite minimum rate of speed prevents detection of slow moving objects.
It would be advantageous to have a vehicle detection system that is failsafe and detects the presence of trains whether stopped, or moving at any speed. It would be further advantageous to have such a system available at a reduced cost as compared to conventional systems.